Method for predictive control of the orientation of a solar tracker

ABSTRACT

A method for controlling the orientation of a single-axis solar tracker orientable about an axis of rotation, including
         observing the evolution over time of the cloud coverage above the solar tracker;   determining the evolution over time of an optimum inclination angle of the solar tracker substantially corresponding to a maximum of solar radiation on the solar tracker, depending on the observed cloud coverage;   predicting the future evolution of the cloud coverage based on the observed prior evolution of the cloud coverage;   calculating the future evolution of the optimum inclination angle according to the prediction of the future evolution of the cloud coverage;   servo-controlling the orientation of the solar tracker according to the prior evolution of the optimum inclination angle and depending on the future evolution of the optimum inclination angle.

TECHNICAL FIELD

The present invention relates to a method for controlling theorientation of a single-axis solar tracker, as well as a single-axissolar tracker designed to implement such a method.

The invention concerns the field of solar trackers, also called trackersupport systems, provided to support solar collectors, generally of thephotovoltaic panel type.

More particularly, it concerns the solar trackers of the single-axistype that is to say orientable according to one single main axis ofrotation, for a rotation allowing tracking the Sun during its rise andits descent from east to west. For accuracy, such a main axis ofrotation generally extends horizontally and substantially parallel tothe ground on which the solar tracker is anchored.

BACKGROUND

In this field, it is common to servo-control the orientation of thesolar tracker based on an astronomical calculation of the position ofthe Sun, for a real-time positioning facing the Sun.

However, this servo-control type has a major drawback by offering ayield deficit under certain meteorological conditions, and we willadvantageously refer to FIG. 1 for explanation; this FIG. 1 includingfour diagrams (1 a), (1 b), (1 c) and (1 d) each illustrating two solartrackers ST under different meteorological conditions, and with the SunSO always at the same position.

The diagram (1 a) illustrates ideal meteorological conditions, in theabsence of clouds, and the solar trackers ST are orientated facing theSun SO in order to benefit from a maximum direct solar radiation Rdir.Under these optimum conditions with a zero-cloud coverage, theservo-control on the position of the Sun SO provides a maximumoperation; such a servo-control corresponding to a servo-control of theorientation of the solar tracker at an inclination angle called directinclination angle defined by the direction of the direct solar radiationRdir at the solar tracker.

The diagrams (1 b), (1 c) and (1 d) illustrate degraded meteorologicalconditions, with different cloud coverages depending in particular onthe cloudy surface or overcast surface, on the types of present cloudsNU, on the number and the position of the clouds NU in front of the SunSO.

Under such cloudy conditions, the servo-control on the position of theSun SO may not provide the best yield, when not considering the diffusesolar radiation Rdif. The diffuse solar radiation Rdif arises when thedirect solar radiation Rdir is dispersed in the clouds NU and theatmospheric particles. The diffuse solar radiation Rdif results from thediffraction of light by the clouds NU and by the various molecules insuspension in the atmosphere. Hence, the diffuse solar radiation Rdifdoes not necessarily follow the direction defined by the Sun SO in thedirection of the observation point at the Earth's surface.

Consequently, under cloudy conditions, it may be preferable, in order toobtain a maximum yield with regards to these conditions, to orientatethe solar trackers ST in an orientation called indirect or diffuseorientation according to a direction of the diffuse solar radiation Rdifwhich does not necessarily correspond to the direction of the directsolar radiation Rdir; such a servo-control corresponding to aservo-control of the orientation of the solar tracker on an inclinationangle called diffuse inclination angle defined by the direction of thediffuse solar radiation Rdif at the solar tracker.

In the diagrams (1 b), (1 c) and (1 d), all the solar trackers ST areprecisely orientated according to diffuse inclination angles distinctfrom the direct inclination angle, in order to offer an optimum yield.

Thus, those skilled in the art would be inclined to servo-control, inreal-time, the orientation of the solar tracker on an optimuminclination angle corresponding to a maximum solar radiation. In theabsence of clouds, the optimum inclination angle would correspond to thedirect inclination angle and, in the presence of a cloudy coverage andeven one single cloud in front of the Sun, the optimum inclination anglewould correspond to a diffuse inclination angle. For this purpose, itwould be sufficient to measure the magnitude of the radiation atdifferent directions, and establish the direction corresponding to amaximum magnitude in order to deduce the optimum inclination angletherefrom.

However, proceeding in this manner would have numerous drawbacks, all ofthem relating to the variation of the diffuse solar radiation over time.Indeed, depending on the time evolution of the cloud layer (because ofthe displacement of the clouds under the effect of the winds) anddepending on the composition of this cloud layer (number, dimensions,location and types of clouds), the diffuse solar radiation may vary moreor less rapidly and therefore the optimum inclination angle may varymore or less quickly over time.

Thus, by servo-controlling the orientation of the solar tracker at thisoptimum inclination angle, the solar tracker may be brought to changeits orientation more or less frequently and more or less quickly. Yet,each orientation change urges at least one actuator (an electric motorin general), generating an electrical consumption and wear of themechanical members loaded by the orientation change (bearings, rotationguide elements . . . ). These electrical consumptions and these wearswill not necessarily be compensated by the gains in productivity whenswitching in real-time to the optimum inclination angle.

As example, starting from an initial situation where the optimuminclination angle corresponds to the direct inclination angle, becauseof the absence of clouds between the Sun and the solar tracker, if onesingle cloud passes in front of the Sun for a few minutes, the optimuminclination angle will be modified during these few minutes beforereturning afterwards to the direct inclination angle. In this case,servo-controlling in real-time the orientation of the solar tracker onthe optimum inclination angle would lead to displacing the solar trackerduring these few minutes, for a benefit which is certainly very littlewith regards to the electrical consumption of the actuator(s) and to thewear.

BRIEF SUMMARY

The present invention aims at solving these drawbacks by proposing amethod for controlling the orientation of a single-axis solar tracker,implementing a step of prediction of the evolution of the cloud coveragein order not to systematically servo-control the orientation of thesolar tracker on the optimum inclination angle, but to anticipate theevolution of this optimum inclination angle to apply an advantageoustrade-off between the gains in solar energy productivity and theelectrical energy losses of the actuator(s), and possibly by taking intoaccount the wear generated by the orientation changes.

To this end, it proposes a method for controlling the orientation of asingle-axis solar tracker orientable about an axis of rotation, saidmethod implementing the following steps:

a) observing the evolution over time of the cloud coverage above thesolar tracker, by observing the cloud coverage at several consecutiveinstants by means of an observation system of the sky above the solartracker;

b) translating each observation performed by the observation system intoa mapping of the solar luminance and determining the evolution over timeof an optimum inclination angle of the solar tracker correspondingsubstantially to the maximum solar radiation on the solar tracker,depending on the observed cloud coverage, by calculating for eachmapping at each instant an optimum inclination angle associated to themaximum solar luminance on said mapping;

c) predicting the future evolution of the cloud coverage based on theprior observed evolution of the cloud coverage, by calculating at eachpresent instant at least a predictive mapping of the solar luminance ata future instant, by implementing a predictive calculation taking intoaccount an evolution of the distribution of the solar luminance on themappings established at several past instants and a speed of evolutionof the solar luminance between said mapping established at several pastinstants;

d) calculating the future evolution of the optimum inclination angledepending on the prediction of the future evolution of the cloudcoverage, by calculating for each predictive mapping a predictiveoptimum inclination angle at a future instant associated to the maximumof the solar luminance on said predictive mapping;

e) servo-controlling the orientation of the solar tracker depending onthe prior evolution of the optimum inclination angle and depending onthe future evolution of the optimum inclination angle.

Thus, the method implements a prediction, in the relatively short term,of the future evolution of the optimum inclination angle; it should berecalled that this optimum inclination angle corresponding to theinclination angle of the solar tracker which offers the highest solarluminance depending on the cloud coverage, this optimum inclinationangle may correspond to either the direct inclination angle (inparticular in the absence of clouds, or at least in the absence of cloudbetween the Sun and the solar tracker), or to a diffuse inclinationangle which will depend on the comparison of the cloud layer (number,dimensions and types of clouds, location of clouds, cloudy surface).

Then, depending on the future evolution of the optimum inclinationangle, a servo-control of the orientation of the solar tracker can beimplemented by anticipation, without following directly and in real-timethe optimum inclination angle, thus allowing avoiding orientationchanges which would result in a in little energy gain, and even whichwould result in energy losses, as it would be the case for example ifone single cloud passes in front of the Sun for a reduced time.

Furthermore, the observation of the cloud coverage is translated into amapping of the solar luminance and this mapping is used to determine theoptimum inclination angle. The mapping corresponds, indeed, to adistribution of the luminance according to several elevation angles(angles generally measured relative to a vertical axis, these elevationangles are to be directly matched to the inclination angle of the solartracker), and the research for the optimum inclination angle correspondsto the research for an elevation angle associated to the maximum solarluminance in the mapping.

It is possible to consider that the mapping is unidimensional, in otherwords with a distribution of the luminance only according to severalelevation angles, or the mapping may be bidimensional in other wordswith a distribution of the luminance according to several elevationangles and also according to several azimuth angles (thus allowingtaking into account the width of the solar collector—dimension takenalong a direction orthogonal to the axis of rotation—and not only thelength of the solar collector—dimension taken along the direction of theaxis of rotation—).

Thus, observation of the cloud coverage is converted into a mapping ofthe solar luminance received by an observation system, this solarluminance varying according to the observation angle. Over time, themappings allow observing the evolution of the solar luminance (directlydependent on the evolution of the cloud layer) and then establishingcartographic predictions on the future state of the mapping of the solarluminance, and therefore the inclination angle which would allow thesolar tracker to benefit, in the future, from a maximum luminance.

According to another feature, during step a), each mapping is abidimensional mapping established according to two directions, a firstand a second directions, and, during step b), the optimum inclinationangle is calculated for each mapping by implementing the followingsteps:

-   -   the mapping constitutes a solar luminance map distributed        according to strips parallel to the first direction and        associated respectively to different elevation angles and        according to columns parallel to the second direction and        associated respectively to different azimuth angles, to each        strip accordingly corresponding an inclination angle of the        solar tracker;    -   calculating for each strip an equivalent luminance value from        the set of luminance values taken in the strip;    -   calculating for each strip a luminance value perceived by the        tracker (1) support from equivalent luminance values calculated        for the set of strips and from inclination angles associated to        the strips;    -   retaining the optimum inclination angle as the inclination angle        associated to the strip having the highest perceived luminance        value.

In this manner, the calculation of the optimum inclination angle isbased on the calculation of perceived luminance values associated toeach strip and thus to each inclination angle.

Advantageously, the first direction is parallel to the axis of rotationof the solar tracker and the second direction is horizontal andorthogonal to the first direction.

Furthermore, the observation of the cloud coverage can be carried outaccording to any of the following methods:

-   -   sky images pickup from the ground by means of an image pickup        apparatus, such as a camera;    -   measurement of the solar luminance from the ground by means of a        set of several photosensitive cells;    -   recovery of satellite images from the sky above the solar        tracker.

With an image pickup, the observation corresponds to an image. With ameasurement of the solar luminance by photosensitive cells, theobservation corresponds to a matrix of the measurements performedindividually by each photosensitive cell, these photosensitive cellsbeing positioned at different elevation angles, and in particulardistributed on a full dome shaped support in order to provide a wideobservation from the sky. With a recovery of satellites images, theobservation corresponds to a satellite image of the concerned area.

According to an advantageous feature, during step b), is implemented astep of frequency weighting applied on the observation which depends onboth a frequency response of the observation system and on the usefulfrequency band of the solar collector.

Thus, the frequency weighting will consist in applying a frequencyfilter which will take into account the spectral response of the solarcollector.

In a particular embodiment, to carry out step a), the cloud coverage isobserved periodically for several consecutive instants, the instant tcorresponding to the present instant and the duration P corresponding tothe time period between two consecutive observations, so that at eachinstant t is associated a mapping;

-   -   to carry out step c), at each present instant t, at least a        predictive mapping at a future instant t+nP, where n is non-zero        integer, is calculated at least from the mappings established at        several past instants t−mP, where m is a non-zero integer;    -   to carry out step d), for each predictive mapping at a future        instant t+nP is calculated a predictive optimum inclination        angle (θopt).

Advantageously, during step d), the optimum inclination angle for thepredictive mapping is calculated according to a calculation methodequivalent to that used during step b) for calculating the optimuminclination angle for a mapping.

In other words, the same type of calculation is implemented to determinethe optimum inclination angle, whether for observations actuallyperformed or for the predictions which are derived from a predictivecalculation in order to improve a match between calculations.

According to one possibility of the invention, during step e), theservo-control of the orientation of the solar tracker is also carriedout according to the energy consumption necessary to modify theorientation of the solar tracker starting from a present inclinationangle until reaching a predictive optimum inclination angle establishedat a future instant during step d).

In other words, the effective servo-control takes into account thisenergy consumption to implement or not an orientation according to afuture (or predictive) optimum inclination angle in order to anticipatea change in cloud coverage.

In accordance with another feature of the invention, during step e), isestablished a potential scenario during which the inclination angle ofthe solar tracker is modified starting from a present inclination angleuntil reaching a predictive optimum inclination angle at a futureinstant established during step d), and to this potential scenario areassociated the calculations of:

-   -   the evolution of the inclination angle of the solar tracker        during the orientation change starting from a present        inclination angle until reaching a predictive optimum        inclination angle, this evolution depending on the speed of        displacement in rotation of the solar tracker;    -   the evolution of the energy consumption necessary to modify the        orientation of the solar tracker;    -   the evolution of the additional solar energy production expected        with such an orientation change;    -   the evolution of the expected energy yield based on the        difference between the solar energy production and the energy        consumption;

then the orientation of the solar tracker is servo-controlled on saidpredictive optimum inclination angle if the energy yield is generallypositive for the scenario, else the orientation of the solar tracker isheld at the present inclination angle.

Thus, the servo-control according to such a predictive (or future)inclination angle will be performed only under an energy benefit, inorder not to implement systematic orientation change at each change inthe cloud coverage.

According to another possibility of the invention, during step e), theservo-control of the orientation of the solar tracker is also carriedout depending on an inclination angle called direct inclination angleestablished by an astronomical calculation of the Sun's position.

It is indeed interesting to take into account this direct inclinationangle, to know the Sun's position and thus to consider a directorientation facing the Sun in the servo-control logic of step e).

In this case, the present orientation angle of the aforementionedscenario can correspond to the direct inclination angle, and theservo-control will take into account the potential energy yieldcalculated in case of orientation change of the direct inclination angleto the predictive optimum inclination angle.

According to another possibility of the invention, during step e), theservo-control of the orientation of the solar tracker is also carriedout depending on a wear rate of mechanical members of the solar trackerloaded during an orientation change of the solar tracker, starting froma present inclination angle until reaching a predictive optimuminclination angle established at a future instant during step d).

The invention also relates to a single-axis solar tracker orientableabout an axis of rotation, of the type comprising a fixed structure foranchorage to the ground and a specific platform capable of supporting atleast one solar collector, said platform being rotatably actuatable onthe fixed structure about said axis of rotation by means of an actuationsystem, said solar tracker being noteworthy in that it further comprisesan observation system of the evolution over time of the cloud coverageabove the solar tracker and a control unit connected, on the one hand,to the observation system in order to receive its observations data and,on the other hand, to the actuation system in order to control therotation of the platform, where said control unit is adapted toimplement the steps b) to e) of the controlling method as describedhereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear uponreading the detailed description hereinafter, of non-limiting examplesof implementation, made with reference to the appended figures in which:

FIG. 1, already discussed, comprises four diagrams (1 a), (1 b), (1 c)and (1 d) each illustrating two solar trackers under differentmeteorological conditions;

FIG. 2 is a schematic view of a single-axis solar tracker in accordancewith the invention, where are illustrated the fixed structure andmovable platform assembly and a system for observing the evolution overtime of the cloud coverage;

FIG. 3 is a schematic view of a first example of observation system;

FIG. 4 is a flat down schematic representation of a first observationsystem equivalent to that of FIG. 3, and of a mapping of the solarluminance (to the right) derived from such an observation system;

FIG. 5 is a schematic view of a second example of observation system;

FIG. 6 is a schematic representation of an observation (at the top left)performed by an observation system equivalent to that of FIG. 5, and ofa mapping of the solar luminance (at the bottom left) derived from suchan observation, after several image processing steps, and of anequivalent matrix of luminance values (at the bottom right) derived fromthis mapping;

FIG. 7a is a schematic side view of four columns of a mapping of thesolar luminance, with the azimuth angles associated to the differentcolumns, in order to illustrate the calculation implemented forcalculating an equivalent luminance value serving to determine theoptimum inclination angle;

FIG. 7b is a schematic side view of four strips of a mapping of thesolar luminance, with the elevation or inclination angles associated tothe different strips, in order to illustrate the calculation implementedfor calculating the perceived luminance value serving to determine theoptimum inclination angle;

FIG. 8 comprises four diagrams (8 a), (8 b), (8 c) and (8 d) eachillustrating an image with the representation of a cloud observed at apast instant (t−2), of the same image observed at a past instant (t−1),of the same image observed at a present instant (t) and of the sameimage predicted by predictive calculation at a future instant (t+1);

FIG. 9 shows three mappings of the solar luminance, to which areassociated the corresponding optimum inclination angles, including amapping at a present instant (t) and two predictions of mapping atfuture instants (t+1) and (t+n);

FIG. 10 is a representation in the form of a functional diagram of thefunctional elements used for the implementation of a controlling methodin accordance with the invention.

FIG. 11 shows five predictive curves calculated for a first potentialscenario defined during the servo-control step e), with from the top tothe bottom, an evolution curve of the future (or predictive) optimuminclination angle calculated during step d), an evolution curve of theinclination angle of the solar tracker, an evolution curve of the energyconsumption necessary to modify the orientation of the solar tracker, anevolution curve of the expected additional solar energy consumption, andan evolution curve of the expected energy yield;

FIG. 12 shows five predictive curves (similar to those of FIG. 11)calculated for a second potential scenario

DETAILED DESCRIPTION

Referring to FIG. 2, a single-axis solar tracker 1 orientable about anaxis of rotation A, of the type comprising a fixed structure 11 foranchorage to the ground constituted by one or several pylon(s) anchoredto the ground, for example by pile driving, screwing, bolting,ballasting, or any other equivalent means allowing fastening andstabilizing the fixed structure 11 to the ground.

The solar tracker 1 further comprises a movable platform 12 rotatablymounted on the fixed structure 11 about the axis of rotation A, and morespecifically rotatably mounted on the upper ends of the pylon(s). Thisplatform 12 is specific to support at least one solar collector 13, andin particular one or several photovoltaic panel(s).

Referring to FIG. 2 and to FIGS. 7a and 7b , the axis of rotation A issubstantially horizontal and directed according to a longitudinal axis Xaccording to the north-south direction. When the solar tracker 1 is flatdown (as shown in FIGS. 2, 7 a and 7 b) with its platform 12 layinghorizontally, the platform 12 extends according to a horizontal planedefined by the longitudinal axis X and by a transverse axis Y accordingto the east-west direction, orthogonally to a vertical axis Z.

In the following description, the inclination angle of the solar tracker1 (or inclination angle of the platform 12 and of the solar collector(s)13) corresponds to the angle of the normal to the platform 12 withrespect to the vertical axis Z considered in the plane (Y, Z). Thus,when the solar tracker 1 is flat down, this inclination angle is 0degree.

The solar tracker 1 also comprises an observation system 2 of the cloudcoverage above the solar tracker 1 or, in other words for observing thesky above the solar tracker 1. This observation system 2 may beassociated to one single solar tracker 1 or, for economic reasons, maybe shared with several solar trackers.

The observation system 2 is fixed, and may be raised with respect to theground, for example by being mounted on a post 20.

The solar tracker 1 further comprises an actuation system (notillustrated in FIG. 2 and bearing the reference number 3 in FIG. 10)which ensures rotating the platform 12 about the axis of rotation A.

This actuation system 3 comprises an actuator, for example of the(electric, pneumatic or hydraulic) cylinder type or of the electricmotor (for example rotary motor) type. This actuation system 3 furthercomprises a mechanical system for transmitting the movement at theoutput of the actuator (a rotational movement for a rotary motor, or alinear movement for a cylinder) into a rotational movement of theplatform 12. As a non-limiting example, this mechanical transmissionsystem may be a deformable-parallelogram system, a pulley system, apinion system, a chain system, a belt system, a clutch system, atransmission shaft system, a connecting rod system, etc.

It is possible to consider that the actuator is specific to the solartracker 1, or is shared between several solar trackers. In the casewhere the actuator is shared, the platforms 12 of the different solartrackers are advantageously coupled in rotation, for a synchronousrotation under the effect of the common actuator.

Referring to FIG. 10, the solar tracker 1 also comprises a control unit4 such as an electronic board, which is connected to the observationsystem 2 to receive its observations (or observations data) and which isalso connected to the actuation system 3 in order to control itsoperation and accordingly control the rotation of the platform 12, inother words the orientation of the solar tracker 1.

This control unit 4 comprises several modules, namely:

-   -   a cartographic module 40 provided to translate or convert each        observation performed by the observation system 2 into a mapping        of the solar luminance, by associating each mapping to a time        instant;    -   an archiving module 41 which archives each mapping generated by        the cartographic module 40;    -   a predictive calculation module 42 which calculates future        evolution of the cloud coverage based on the prior observed        evolution of the cloud coverage, and more precisely calculates        predictive mappings of the solar luminance for future instants,        this predictive calculation module 42 carrying out these        calculations based on the mappings generated in real-time by the        cartographic module 40 and based on the past mappings archived        in the archiving module 41;    -   an optimum inclination angle calculation module 43 which        calculates the optimum inclination angle for each mapping        generated in real-time by the cartographic module 40 (that is to        say the optimum inclination angle at a present instant) and for        each predictive mapping derived from the predictive calculation        module 42 (in other words the optimum inclination angles for        future instants);    -   an optimum inclination angle evolution module 44 which recovers        all optimum inclination angles derived from the optimum        inclination angle calculation module 43 in order to establish        the evolution of the optimum inclination angle, and therefore to        predict and anticipate the changes of the optimum inclination        angle;    -   a parametrization module of the solar tracker 45 which comprises        parameters relating to the speed of displacement of the        actuation system 3 (and therefore to the speed necessary for an        orientation change), parameters relating to the energy        consumption necessary for the actuation system 3 for an        orientation change, parameters relating to the solar energy        production generated by the one or several solar collector(s) 13        depending on the received solar luminance, and parameters        relating to a wear rate of the mechanical members of the solar        tracker 1 loaded during an orientation change of the solar        tracker 1, these parameters being in particular dependent on the        angular deviation between the beginning and the end of an        orientation change;    -   an astronomical calculation module 46 which calculates in        real-time the Sun's position, and therefore the direct        inclination angle defined by the direct solar radiation at the        solar tracker 1;    -   a servo-control module 47 which calculates the servo-control of        the orientation of the solar tracker 1, in other words the        servo-control of its inclination angle depending on the        evolution of the optimum inclination angle coming from the        module 44, on the different parameters coming from the module 45        and on the direct inclination angle coming from module 46, where        this servo-control module 47 outputs an angular setpoint to the        actuation system 3 in order to control the orientation changes        of the platform 12 of the solar tracker 1.

It should be noted that this control unit 4 may be specific to the solartracker 1, or shared between several solar trackers, and preferablybetween several solar trackers arranged in line (extending from north tosouth) within linear solar plants.

In a first embodiment illustrated in FIG. 3, the observation system 2comprises a support 21, in particular a full dome shaped support,supporting photosensitive cells 22.

These photosensitive cells 22 are positioned along several strips (orlines) distributed at several angles called elevation angles θi whichare measured with respect to the vertical axis Z in the plane (Y, Z),the reference frame (X, Y, Z) being centered on the center O of the fulldome 21. Hence, the elevation angle θi is to be matched with theinclination angle of the solar tracker 1. In the example of FIG. 3, thephotosensitive cells 22 are distributed according to six strips atelevation angles of 0, +θ1, +θ2, +—03, −θ1, −θ2 and −θ3; for examplewith [θ1]=30 degrees, [θ2]=60 degrees and [θ3]=90 degrees. Theseelevation angles θi are also shown in FIG. 7 b.

On each strip are present one or even several photosensitive cells 22,In case of a strip with several photosensitive cells, the photosensitivecells 22 of the same strip are distributed according to several anglescalled azimuth angles Rj which are measured with respect to the verticalaxis Z in the plane (X, Z). Thus, besides being distributed along thestrips at different elevation angles θi, the photosensitive cells 22 arealso distributed according to columns at different azimuth angles Rj.These azimuth angles Rj are shown in FIG. 7 a.

Generally, the more the first observation system 2 comprisesphotosensitive cells 22, and in particular the more the observationsystem 2 comprises strips of photosensitive cells 22, the better will bethe resolution of the angular accuracy.

These photosensitive cells 22 may be of the same technology as thephotovoltaic panels 13 in order to enable the application of a weightingdepending on the useful wavelength range of the photovoltaic panels 13.Preferably, these photosensitive cells 22 will undergo a priorcalibration in order to obtain a better accuracy.

Thus, with such an observation system 2, by recovering the measurementsof the luminosity of each photosensitive cell 22 and by knowing theelevation angles θi (or associated inclination angles) of the differentstrips and the azimuth angles Rj of the different columns, thecartographic module 40 converts an observation performed by theobservation system 2 into a mapping 5 of the solar luminance.

This mapping 5 constitutes a bidimensional mapping in the sense that itforms a solar luminance map (or matrix) distributed according to:

-   -   several strips 50(i) (i being an integer) established according        to a first direction parallel to the axis of rotation A        (therefore parallel to the axis X), and associated respectively        to different elevation or inclination angles θi, so that each        strip 50(i) corresponds to an inclination angle θi of the solar        tracker 1 and    -   several columns 51(j) (j being an integer) established according        to a second direction horizontal and orthogonal to the axis of        rotation A (and therefore parallel to the axis Y) and associated        respectively to different azimuth angles Rj.

Thus, the mapping 5 comprises N boxes (where N corresponds to the numberof photosensitive cells 22 and N=[i×j]) and an (absolute or relative)solar luminance value Lum(i, j) corresponds to each box.

In FIG. 4 to the left, an example of the first observation system 2 isschematically illustrated flat down and comprises nine photosensitivecells 22 distributed according to three strips B1, B2, B3 which areassociated to three elevation angles (or inclination angles), andaccording to three columns C1, C2, C3 which are associated to threeazimuth angles. To this first observation system 2 corresponds a mapping5 with three strips 50(1), 50(2), 50(3) and three columns 51(1), 51(2),51(3), and where the solar luminance values are expressed in a relativemanner as a percentage.

In a second embodiment illustrated in FIG. 5, the observation system 2comprises a camera, in particular a hemispherical camera, in order toextract images from the sky.

Advantageously, the second observation system 2 (called camera in thefollowing) is adapted to take images in a spectral band sufficient forthe technology of the solar collectors 13, and in particular of thephotovoltaic panel(s).

Referring to FIG. 6, the camera 2 delivers a raw image IMB of the skywhich is delivered afterwards to the cartographic module 40 to convertthis raw image IMB (or observation) into a bidimensional mapping 5 ofthe solar luminance. A reference frame (X, Y) is associated to thisbidimensional raw image IMB, these axes X and Y having already beendefined hereinbefore.

The cartographic module 40 implements a succession of image processingsteps starting from the raw image IMB until the mapping 5.

At a first step POND, the cartographic module 40 implements a frequencyweighting applied on the recovered raw image IMB (or video signal), inorder to obtain an image called weighted image IMP; this frequencyweighting consisting in applying a frequency filter on the observation(whether the observation performed by the photosensitive cells 22 orperformed by the camera) which depends on both the frequency response ofthe observation system 2 (whether the photosensitive cells 22 or thecamera) and the useful frequency band (or spectral response) of thephotovoltaic panels 13.

At a second step TRAIT, the cartographic module 40 implements aprocessing of the weighted image IMP consisting in correcting the imagefrom defects (noise suppression processing, blooming processing,saturation processing . . . ) in order to obtain an image calledprocessed image IMT. Then, the cartographic module 40 implements acalculation (whether pixel by pixel, or area by area where each areacomprises several pixels) of the distribution of the solar luminance onthe processed image IMT in order to generate an initial mapping CIforming a map (or matrix) of solar luminance distributed according toseveral strips associated respectively to different elevation orinclination angles θ(i) and according to several columns associatedrespectively to different azimuth angles; such an initial mapping beingequivalent to that already described hereinabove. In FIG. 6, the solarluminance values of the initial mapping CI are expressed in a relativemanner as a percentage.

At a third step SENS, the mapping module 40 applies on the initialmapping CI a coefficient depending on the variation of the sensitivityof the camera 2, in order to generate a mapping 5 of the same type asthe mapping described hereinabove. Indeed, the magnitude (or luminosity)of the data delivered by the camera 2 is proportionally related to thevalue of the solar radiation, so that this coefficient takes intoaccount this proportionality depending on the variation of thesensitivity of the camera 2.

Thus, the mapping module 40 generates a mapping 5 forming a map (ormatrix) of solar luminance distributed according to several strips 50(i)associated respectively to different elevation or inclination angles θiand according to several columns 51(j) associated respectively todifferent azimuth angles Rj. In the example of FIG. 6, the mapping 5comprises five strips 50(1), . . . , 50(5) and seven columns 51(1), . .. , 51(7), and the solar luminance values are expressed in a relativemanner as a percentage.

The resolution of the mapping 5 (in other words the number of strips andcolumns) and therefore the angular accuracy depend on the fineness ofthe image processing implemented by the cartographic module 40, and alsoon the sensitivity and on the resolution of the observation system 2.For the first observation system 2 with photosensitive cells 22, thissensitivity depends on the sensitivity of the photosensitive cells 22,and this resolution depends on the number and on the distribution of thephotosensitive cells 22. For the second observation system 2 of thecamera type, this sensitivity and this resolution depend on the qualityof the camera.

Starting from such a mapping 5 (whether it is derived from either one ofthe observation systems 2 described hereinabove), the optimuminclination angle calculation module 43 implements a calculation basedon this mapping 5 in order to extract an optimum inclination angle(θopt) which corresponds to the inclination angle (or elevation angle)to which is associated a maximum of solar luminance.

For this calculation, and with reference to FIGS. 6 and 7, the optimuminclination angle calculation module 43 implements a succession ofsub-steps. This succession of sub-steps constitutes an example ofcalculation or algorithm mode, and the invention would not of course berestricted to this example.

In a first sub-step, the optimum inclination angle calculation module 43calculates, for each strip 50(i) of the mapping 5, an equivalentluminance value Leq(i) from the set of luminance values L(i, j) taken inthe strip 50(i). For each strip 50(i), the equivalent luminance valueLeq(i) of the strip 50(i) is a function of the luminance values L(i, j)taken in the strip 50(i) and of the azimuth angles Rj of the differentcolumns 51(j) according to the following formula (with reference to FIG.7a ).

${{Leq}(i)} = {\sum\limits_{j}{{{Lum}\left( {i,j} \right)} \times \cos \; {Rj}}}$

A matrix MLeq of the equivalent luminance values Leq(i) associated tothe different strips 50(i) is thus obtained.

In a second sub-step, the optimum inclination angle calculation module43 calculates, for each strip 50(i) of the mapping 5, a perceivedluminance value Lperc(i) by the tracker support 1 from equivalentluminance values Leq(i) calculated for all strips during the firstsub-step, and from the inclination (or elevation) angles θi associatedto the strips 50(i), according to the following formula (with referenceto FIG. 7b ):

${{Lperc}(i)} = {\sum\limits_{k}{{{Leq}(k)} \times {\cos \left( {{\theta \; i} - {\theta \; k}} \right)} \times {p\left( {i,k} \right)}}}$

where p(i, k)=1 if abs(θi−θk)<90 degrees, and p(i, k)=0 else.

The coefficient takes into account, beyond an angular deviation of 90degrees, the radiation is not received by the plane solar collector(s).

Thus, a matrix MLperc of the perceived luminance values Lperc(i)associated with the different strips 50(i) is obtained.

In a last sub-step, the optimum inclination angle calculation module 43retains the optimum inclination angle θopt as the inclination (orelevation) angle associated to the strip having the highest perceivedluminance value Lperc(i).

The predictive calculation module 42 calculates the predictive mappings6 of the solar luminance for future instants (t+nP), where n is anon-zero integer and P the period of observation performed periodicallyand repeatedly by the observation system 2. These predictive mappings 6are established based on the mapping 5 generated in real-time by thecartographic module 40 and based on the past mappings 5 archived in thearchiving module 41.

FIG. 8 illustrates four examples of situation of a cloud coverageevolving therein, with four diagrams 8 a, 8 b, 8 c and 8 d eachrepresenting an image with the representation of a cloud observed at apast instant (t−2), of the same cloud observed at a past instant (t−1),of the same cloud observed at a present instant (t) and of the samecloud predicted by predictive calculation at a future instant (t+1) (theperiod P is 1 in FIG. 8).

The predictive calculation is based on a consideration of the pastevolution of the solar luminance, between several past instants and thepresent instant, and in particular the evolution of the distribution ofthe solar luminance and the speed of evolution of the solar luminance.

This predictive calculation can be based on a sliding time window, thatis to say a window comprising a predefined number of the last pastmappings.

This predictive calculation is used to establish short-term predictivemappings 6 (or predictions of mapping). As a non-limiting example, theshort-term concept covers calculations on a future horizon of a maximumof ten to thirty minutes, or even a maximum of one to two hours. It isof course possible to provide longer-term predictive calculations.

The algorithm implemented for such a predictive calculation may possiblyintegrate improvements such as:

-   -   the consideration of the prediction errors to improve the future        predictions (indeed, it is possible to compare these mappings        with the cartographic predictions performed earlier in order to        draw lessons therefrom on the predictive calculation and to        improve it);    -   identifying the types of cloud depending on the mapping thanks        to a database and/or thanks to analyses and plots performed in        the past, so as to allow making long-term predictions depending        on the types of clouds.

The algorithm implemented for such a predictive calculation may alsotake into account the evolution of the Sun's position in the sky, inparticular when the predictive calculation is performed at futureinstants sufficiently distant (for example beyond 30 minutes) so thatthe change of the Sun's position has an influence on the evolution ofthe solar luminance. This consideration of the Sun's position in thepredictive calculation is illustrated by the linking arrow in dottedline in FIG. 10 between the predictive calculation module 42 and theastronomical calculation module 46.

As shown in FIG. 9, the predictive calculation module 42 establishespredictive mappings 6, and to each predictive mapping 6 is associated apredictive optimum inclination angle θopt calculated by the optimuminclination angle calculation module 43 using the same calculationmethod described above.

Thus, the optimum inclination angle evolution module 44 recovers all theoptimum inclination angles (those of the past mappings, those of thepresent mapping, and those of the predictive mappings 6) and establishesa future evolution of the optimum inclination angle θopt and thus topredict and anticipate the changes of the optimum inclination angle

Finally, the servo-control module 47 servo-controls the orientation ofthe solar tracker 1 depending on the past and future evolution of theoptimum inclination angle θopt, and also depending on the energyconsumption Cons necessary to modify the orientation of the solartracker 1, on the speed of displacement in rotation of the solar tracker1, and the production of additional solar energy Prod obtained with anorientation change.

With reference to FIGS. 11 and 12, the servo-control module 47 is basedon the future evolution of the optimum inclination angle θopt (firstcurve from the top).

In the given example, the predictive optimum inclination angle θoptchanges in value to reach a target value θc, for example due to aprediction of passage of a cloud in front of the Sun, from the futureinstant t1 to a future instant t2, before returning to its initialvalue.

The servo-control module 47 establishes a potential scenario duringwhich the inclination angle θ of the solar tracker 1 is modifiedstarting from a present inclination angle θp until reaching the futureor predictive optimum inclination angle, in this case following theprediction of evolution of the optimum inclination angle.

In the given example, the scenario consists in servo-controlling theinclination angle θ on the first curve, and this servo-control dependson the speed of displacement in rotation of the solar tracker 1, inorder to obtain a second evolution curve of the inclination angle θ ofthe solar tracker 1 during the orientation change of the scenario.Indeed, the solar tracker 1 has a displacement time necessary to get toreach the target optimum inclination angle θc.

Thanks to the predictive calculation, the displacement of the solartracker 1 is anticipated, in this case by starting earlier at theinstant t10 (prior to t1) until reaching the target value θc at t11(subsequent to t1), then by starting by anticipation the return to theinstant t11 (prior to t2) until returning to the present inclinationangle Op at the instant t13 (subsequent to t2).

The servo-control module 47 determines the evolution of the energyconsumption Cons necessary to modify the orientation of the solartracker according to the second curve, in order to obtain a thirdevolution curve of this energy consumption Cons; the solar tracker 1consuming during the orientation change phase, between the instants t10and t11 then between the instants t12 and t13.

The servo-control module 47 determines the evolution of the additionalproduction Prod (or gain of production) expected following the secondevolution curve of the inclination angle θ rather than remaining at thepresent inclination angle θp, in order to obtain a fourth curve of theevolution of this production Prod. Therefore, this additional productionProd corresponds to the gain of production expected if the scenario isfollowed rather than remaining in the initial or present situation atthis angle θp.

In the given example, the production Prod is negative between theinstants t10 and t1 and between the instants t2 and t13 which correspondto periods where the inclination angle θ moves away from the inclinationangle θopt, and the production Prod is positive between the instants t1and t2 which correspond to a period where the inclination angle θmatches or even is equal to the inclination angle θopt.

The servo-control module 47 determines the evolution of the energy yieldRend expected based on the difference between the production Prod andthe energy consumption Cons, giving a fifth curve corresponding to thedifference between the fourth curve and the third curve, in other wordsRend=Prod−Cons.

In the given example, the yield Rend is negative between the instantst10 and t1 and between the instants t2 and t13, and the yield Rend ispositive between the instants t1 and t2.

Finally, the servo-control module 47 follows the scenario (in otherwords servo-controls the solar tracker according to the second curve) ifthe energy yield is generally positive for the scenario, else theorientation of the solar tracker 1 is held at the inclination angle θp.

The overall energy yield is established by studying the yield throughoutthe scenario period.

In the example of FIG. 11, the overall yield is negative, because thesum of the surfaces Srn where the yield is negative (between t10 and t1and between t2 and t13) is greater than the surface Srp where the yieldis positive (between t1 and t2). The example in FIG. 11, corresponds forexample to a situation where the predictive passage time (correspondingto the interval [t2-t1]) of a cloud in front of the Sun is too shortcompared to the time necessary for an orientation change (correspondingto the interval [t1-t10] or [t13-t2]).

In the example of FIG. 12, the overall yield is positive, because thesum of the surfaces Srn where the yield is negative (between t10 and t1and between t2 and t13) is lower than the surface Srp where the yield ispositive (between t1 and t2). The example of FIG. 12 corresponds forexample to a situation where the predictive passage time (correspondingto the interval [t2-t1]) of a cloud in front of the Sun is long comparedto the time necessary for an orientation change (corresponding to theinterval [t1-t10] or [t13-t2]).

Thus, in the example of FIG. 11, the servo-control module 47 does notfollow the scenario and maintains the orientation at the present valueOp, while in the example of FIG. 12, the servo-control module 47 followsthe scenario and ensures a servo-control of the inclination angleaccording to the second curve.

Of course, the example of implementation mentioned hereinabove is notlimiting and other improvements and details may be added to the solartracker according to the invention, nevertheless without departing fromthe scope of the invention where other types of fixed structure orplatform may be for example carried out.

1. A controlling method for controlling the orientation of a single-axissolar tracker orientable about an axis of rotation, said controllingmethod comprising: a) observing an evolution over time of a cloudcoverage above the solar tracker, by observing the cloud coverage atseveral consecutive instants by means of an observation system of thesky above the solar tracker; b) translating each observation of thecloud coverage performed by the observation system into a mapping of thesolar luminance and determining an evolution over time of an optimuminclination angle of the solar tracker corresponding substantially to amaximum solar radiation on the solar tracker, depending on theobservation of the cloud coverage, by calculating for each mapping ateach instant an optimum inclination angle associated to a maximum solarluminance on said mapping; c) predicting a future evolution of the cloudcoverage based on a prior observed evolution of the cloud coverage, bycalculating at each present instant at least a predictive mapping of thesolar luminance at a future instant, by implementing a predictivecalculation taking into account an evolution of the distribution of thesolar luminance on mappings established at several past instants and aspeed of evolution of the solar luminance between said mappingsestablished at several past instants; d) calculating a future evolutionof the optimum inclination angle depending on a prediction of the futureevolution of the cloud coverage, by calculating for each predictivemapping a predictive optimum inclination angle at a future instantassociated to a maximum of the solar luminance on said predictivemapping; and e) servo-controlling the orientation of the solar trackerdepending on a prior evolution of the optimum inclination angle anddepending on the future evolution of the optimum inclination angle. 2.The controlling method according to claim 1, wherein, during step a),each mapping is a bidimentional mapping established according to twodirections, a first and a second directions, and, during step b), theoptimum inclination angle is calculated for each mapping by implementingthe following steps: the mapping constitutes a solar luminance mapdistributed according to strips parallel to the first direction andassociated respectively to different elevation angles and according tocolumn parallel to the second direction and associated respectively todifferent azimuth angles, to each strip accordingly corresponding aninclination angle of the solar tracker; calculating for each strip anequivalent luminance value from the set of luminance values taken in thestrip; calculating for each strip a luminance value perceived by thesolar tracker from equivalent luminance values calculated for the set ofstrips and from inclination angles associated to the strips; retainingthe optimum inclination angle as the inclination angle associated to thestrip having the highest perceived luminance value.
 3. The controllingmethod according to claim 2, wherein, the first direction is parallel tothe axis of rotation of the solar tracker and the second direction ishorizontal and orthogonal to the first direction.
 4. The controllingmethod according to claim 1, wherein the observation of the cloudcoverage is carried out according to any of the following methods: skyimages pickup from the ground by means of an image pickup apparatus,such as a camera; measurement of the solar luminance from the ground bymeans of a set of several photosensitive cells; recovery of satelliteimages of the sky above the solar tracker.
 5. The controlling methodaccording to claim 1, wherein, during step b), there is implemented afrequency weighting step applied on the observation of the cloudcoverage which is a function of both a frequency response of theobservation system and a useful frequency band of the solar collector.6. The controlling method according claim 1, wherein: to carry out stepa), the cloud coverage is observed periodically at several consecutiveinstants, the instant t corresponding to the present instant and theduration P corresponding to the period of time between two consecutiveobservations, so that to each instant t a mapping is associated; tocarry out step c), at each present instant t, there is calculated atleast a predictive mapping at a future instant t+nP, where n is non-zerointeger, at least from the mappings established at several past instantst−mP, where m is a non-zero integer; to carry out step d), for eachpredictive mapping at a future instant t+nP is calculated a predictiveoptimum inclination angle.
 7. The controlling method according to claim1, wherein, during step d), the optimum inclination angle for apredictive mapping is calculated according to a calculation methodequivalent to that used during step b) for calculating the optimuminclination angle for a mapping.
 8. The controlling method according toclaim 1, wherein, during step e), the servo-control of the orientationof the solar tracker is carried out also depending on an energyconsumption necessary to modify the orientation of the solar tracker bystarting from a present inclination angle until reaching a predictiveoptimum inclination angle established at a future instant during stepd).
 9. The controlling method according to claim 8, wherein, during stepe), there is established a potential scenario during which theinclination angle of the solar tracker is modified starting from apresent inclination angle until reaching a predictive optimuminclination angle at a future instant established during step d), and tothis potential scenario are associated the calculations of: an evolutionof the inclination angle of the solar tracker during the orientationchange starting from a present inclination angle until reaching apredictive optimum inclination angle, this evolution depending on aspeed of displacement in rotation of the solar tracker; an evolution ofthe energy consumption necessary to modify the orientation of the solartracker; an evolution of an additional solar energy production expectedwith such an orientation change; an evolution of an expected energyyield based on the difference between the solar energy production andthe energy consumption; and then the orientation of the solar tracker isservo-controlled on said predictive optimum inclination angle if theenergy yield is generally positive for the potential scenario, else theorientation of the solar tracker is held at the present inclinationangle.
 10. The controlling method according to claim 1, wherein, duringstep e), the servo-control of the orientation of the solar tracker isalso carried out according to a direct inclination angle established byan astronomical calculation of the Sun's position.
 11. The controllingmethod according to claim 9, wherein the present angle of orientationcorresponds to the direct inclination angle.
 12. The controlling methodaccording to claim 1, wherein, during step e), the servo-control of theorientation of the solar tracker is also carried out depending on a wearrate of mechanical members of the solar tracker loaded for anorientation change of the solar tracker by starting from a presentinclination angle until reaching a predictive optimum inclination angleestablished at a future instant during step d).
 13. A single-axis solartracker orientable about an axis of rotation, of the type comprising afixed structure for anchorage to a ground and a platform capable ofsupporting at least one solar collector, said platform being rotatablyactuatable on the fixed structure along said axis of rotation by meansof an actuation system, wherein said solar tracker further comprises anobservation system for observing an evolution over time of a cloudcoverage over the solar tracker and a control unit in connection, on theone hand, with the observation system to receive its observation dataand, on the other hand, with the actuation system for controlling therotation of the platform, where said control unit is configured toimplement the steps b) to e) of the controlling method according toclaim 1.